Waveguides, in particular optical planar waveguides, are important components in semiconductor lasers and optical devices used in communication systems, such as optical switches, couplers, splitters and filters. Planar waveguides may include, for example, a doped silica core formed over a substrate such as a silicon wafer. Typically, a cladding layer, comprised of silicon dioxide, surrounds the core. By virtue of its lower refractive index as compared to the core, the silicon dioxide cladding allows a guided mode of light to form and transmit through the waveguide.
The fabrication of waveguides containing a silicon dioxide cladding may be problematic, however. To function as an effective cladding, the silicon dioxide layer, in some cases, may need to be up to about 15 microns thick. However, expensive speciality equipment is required to produce such thick layers. Additionally, the process for depositing and growing layers of such thickness may take several days. Moreover, for certain devices, a second cladding layer grown above the core waveguide material may take a similar period. As such, planar waveguides containing a silicon dioxide layer are slow and expensive to produce. In addition, silicon dioxide based claddings are prone to cracking and stress-induced changes in refractive index, thus requiring additional procedures to prevent such problems. As an alternative, a number of polymeric materials have been proposed as replacements for silicon dioxide claddings.
To facilitate the optimal performance of a planar waveguide, however, polymeric cladding layers should have several properties. As noted above, the refractive index of the cladding should be lower than the refractive index of the core. Therefore a polymer having a high refractive index (e.g., greater than about 1.6) would severely limit the choice of materials that could be used as the waveguide core. The cladding should also have sufficient thermal and mechanical stability that the formation of additional components requiring the application of heat will not alter the cladding's functionality. Similarly, the cladding should have sufficient thermo-mechanical stability to function during high temperature applications, such as greater than about 175° C. In addition, the cladding should be resistant to stress, cracking and delamination from the substrate or core, all of which may reduce the cladding's ability to prevent light losses. Furthermore, the cladding should be capable of providing a uniform planar interface for the core, so as to reduce losses due to light scattering. Similarly, the cladding layer should have a uniform refractive index throughout. And, the cladding layer should be inexpensive and rapid to fabricate.
Previously proposed polymeric materials, however, fail to possess one or more of these properties. For example, polymeric materials having a refractive index equal to or greater than the refractive index of the core waveguide, are unsuitable for use as a cladding. The use of certain polymers, for example, ultraviolet-cured acrylates, is unfavorable at temperatures greater than about 120° C. Yet other polymers, for example, fluorinated acrylates or poly dimethyl silicones, may have poor adhesive properties with respect to the substrate or core, and therefore prone to delamination.
Still other polymers, for example, certain cyclic olefin copolymers, polycarbonates or certain polyimides, are not liquids at room temperature, and thus need to be dissolved in a solvent to be applied as a cladding layer. This, in turn, may unfavorably limit the thickness of the cladding layer that can be deposited in a single pass. Consequently, additional coating steps may be required, thereby increasing the time and cost to produce the waveguide. It may also be necessary to have additional steps to at least partially remove the solvent prior to curing the polymer. And, the evaporation of residual solvent during the coating process may result in a nonuniform surface, with consequent greater light scattering losses.
In addition, solvent free operations present a number of environmental advantages. For example, the need to comply with environmental regulations associated with the use of certain solvents is avoided. The need for special equipment to ensure adequate ventilation of solvents or to handle solvent wastes is also obviated.
Therefore, previously proposed waveguides having a polymeric cladding layer lack the desired characteristics demanded by today's communications industry. Accordingly, what is needed in the art is a waveguide that meets the stringent requirements of the communications industry, while not experiencing the problems associated with previous waveguides.